Contemporary alternator based charging systems in vehicular applications typically rely on a determination of a state of alternator rotation for changing an operating mode of a regulator. A startup state indicates that the alternator is not rotating--thus stationary. A rotating state indicates that the alternator is rotating--thus providing energy to charge a vehicle's battery. Preferably, during the startup state of the alternator, the regulator is constrained to have a limited duty-cycle to reduce battery discharge current. Also, during the startup state of the alternator, an indicator lamp is illuminated to show that the alternator is stationary.
In a prior art circuit shown in FIG. 1 this alternator rotation state is detected by an alternator phase detection system 139 comprised of a comparator 101 in cooperation with threshold circuitry. The comparator 101 compares a signal generated from alternator stator windings 115, 117, and 119 at a connection terminal 113 scaled by resistors 109 and 111, with a reference signal 103 generated by a voltage threshold generator 107. In operation, when an ignition switch 135 is initially closed, a battery 143 is connected to and powers the alternator phase detection system 139. While the vehicle's engine is stationary the alternator is stationary and the signal generated at connection terminal 113 is ideally zero volts. Responsive to this condition the comparator 101 outputs a startup state of a status signal at its output terminal 137. This startup state is a DC level. When the alternator commences rotation, responsive to the engine starting, the signal generated at connection terminal 113 increases. When the signal generated at connection terminal 113 exceeds a magnitude of the reference signal 103, the comparator 101 outputs a rotating state of the status signal at its output terminal 137. The rotating state is a signal with transitions.
In some prior art circuits the voltage threshold generator 107 is a simple voltage reference. Typically, this may take the form of a band-gap voltage reference circuit. A problem with this approach is that rectifier leakage in rectifiers 121, 123, 125, 127, 129 and 131 can cause a DC voltage to appear at connection terminal 113 while the alternator is stationary. This DC voltage, or error, can be significant. In fact it can exceed the reference signal 103 while the alternator is stationary--thus falsely indicating that the alternator is in a rotating state if the following circuit is level sensitive or a reduced sensitivity if it is transition sensitive.
Other prior art voltage threshold generators use a sample-and-hold circuit in the voltage threshold generator 107. This sample-and-hold circuit samples a voltage at the junction of resistors 109 and 111 provided by the signal at connection terminal 113 before the alternator can start rotating. This sampled voltage is used to determine the threshold signal 103 to account for the above-described rectifier leakage, or DC error. If a DC leakage is present at the stator winding 115, due to the rectifiers 123 and 129, then the resultant DC signal representative of the leakage will dominate the AC signal representative of the stator winding excitation associated with the alternator rotation. This will significantly reduce the effective sensitivity of the circuit 139 because the DC component of the signal will swamp out the AC component of the signal, particularly if the resulting DC voltage provided at terminal 113 is proximate one-half the battery voltage, so that the rectifiers do not conduct until the AC signals related to stator winding excitation are very large--usually only at high alternator rotational speed. In other prior art systems this possible sensitivity loss has been compensated by connecting a comparator between two of the alternator stator-rectifier circuits. This in-effect solves the sensitivity problem but it is expensive and difficult to manufacture because of the extra connections required to get the second stator winding signal to the alternator phase detection system 139. In particular, a wire-bonding pad has to be added to a regulator IC to connect the second signal. This can add significantly to the regulator IC die size, process time, and thus cost.
What is needed is an improved alternator phase detection system that is less sensitive to rectifier leakage effects and is cost effective.